Curable silicone coatings for release of pressure-sensitive adhesives (PSA) are known. These coatings may be cured via tin-catalyzed condensation reactions, platinum-catalyed addition reactions, or radiation-initiated crosslinking reactions.
Linear polydimethyl siloxanes form what are referred to as premium or low release coatings. When crosslinked, these materials form abhesive surfaces from which a PSA-laminated facestock may be detached with very little force required. Such premium release characteristic is desirable for many applications.
There are however, significant applications for silicone release coatings which do not provide easy release from PSA's. Such coatings which provide a tight or controlled release characteristic are desirable, for example, in industrial labeling operations which run at high speed. A high release silicone agent is desirable in order to prevent so-called premature dispensing of the label from the release liner. A controlled release characteristic is useful for two-sided release laminate applications wherein one side of the laminate is coated with a premium release silicone and the other side is coated with a tight release silicone, thus providing a differential release structure.
Ideally, silicone release systems that have a range of release characteristics are desirable. Such silicone coating systems would enable an end-user to selectively adjust the formulation to give the desired level of release depending on the specific application. A tight release polymer capable of giving a range of release levels in a blend with premium release polymer is referred to as a controlled release additive (CRA).
Ultraviolet light-curable silicone release coatings based on epoxysilicone polymers, as taught by Eckberg et al., U.S. Pat. No. 4,279,717, are widely used in the release coating and pressure-sensitive adhesive (PSA) industries. For example, see generally the chapter entitled "Silicones" by B. Hardman and A. Torkelson in the Encyclopedia of Polymer Science and Engineering, 2nd edit., Vol. 15, pp. 204-308, 1989, John Wiley & Sons, Inc., New York. Epoxysilicone polymers are conveniently manufactured through the hydrosilation reaction between an SiH-containing silicone monomer or polymer, and olefin epoxides. The general hydrosilation reaction between a silicone and olefin can be expressed for monofunctional silane derivatives as EQU .tbd.SiH+CH.sub.2 .dbd.CH--Z.fwdarw..tbd.Si--CH.sub.2 --CH.sub.2 --Z
and for di-functional siloxane derivatives as EQU --(--CH.sub.3 (H)SiO--)--+CH.sub.2 .dbd.CH--Z.fwdarw.--(--(CH.sub.3)(ZCH.sub.2 CH.sub.2)SiO--)--
where in both cases Z is an organic radical. The hydrosilation reaction is particularly useful for the addition of functional radicals onto silanes and silicones. For example, reaction of a hydrogensiloxane with an epoxy-containing olefin yields an epoxy-functional siloxane.
Epoxysilicones generated through, for example, the hydrosilation reaction can be cured either thermally or, in the presence of the appropriate catalysts and possibly accelerators, by irradiation. Generally, UV-induced, cationic catalysis is preferred in the cure reaction of epoxysilicones due to the relatively high cure rates achieved, and the low temperature which can be employed, thereby preventing damage to temperature-sensitive materials being coated, and the low risk of potential hazards to both industrial users and the environment. Upon exposure to UV radiation, cationic type photo-initiators generate a strong Bronsted acid, which effects the opening of the oxirane ring in the epoxide radical of an epoxysilicone polymer, and the subsequent etherification through which cross-linking of the resin is achieved.
The curing of epoxysilicone polymers is well documented in the patent literature. For example, U.S. Pat. No. 4,576,999, issued to Eckberg, discloses epoxy and/or acrylic functional polysiloxanes as UV-curable adhesive release coatings. The catalyst may be a photo-initiating onium salt and/or a free radical photo-initiating catalyst. U.S. Pat. Nos. 4,279,717 and 4,421,904, both issued to Eckberg, et al., disclose epoxy functional diorganosiloxane fluids combined with iodonium salts to form UV-curable adhesive release compositions. U.S. Pat. No. 4,547,431 discloses an epoxy functional diorganosiloxane combined with onium salt catalyst and polyfunctional epoxy monomers to also form an adhesive coating.
As described in U.S. Pat. No. 4,576,999, the preferred UV photo-initiators for the curing of epoxysilicones are the "onium" salts, of the general formulae: EQU R.sub.2 I.sup.+ MX.sub.n.sup.- EQU R.sub.3 S.sup.+ MX.sub.n.sup.- EQU R.sub.3 Se.sup.+ MX.sub.n- EQU R.sub.4 P.sup.+ MX.sub.n.sup.- EQU R.sub.4 N.sup.+ MX.sub.n.sup.-
where different radicals represented by R can be the same or different organic radicals from 1 to about 30 carbon atoms, including aromatic carbocyclic radicals from 6 to 20 carbon atoms which can be substituted with from 1 to 4 monovalent radicals selected from C.sub.(1-8) alkoxyl, C.sub.(1-8) alkyl, nitro, chloro, bromo, cyano, carboxy, mercapto, etc., and also including aromatic heterocyclic radicals including, for example, pyridyl, thiopheny, puranyl, and others; and MX.sub.n.sup.- is a non-basic, non-nucleophilic anion, such as BF.sub.4.sup.-, PF.sub.6.sup.-, AsF.sub.6.sup.-, SbF.sub.6.sup.-, HSO.sub.4.sup.-, ClO.sub.4.sup.-, and others as known in the art. The photo-initiators may be mono- or multi-substituted mono, bis or tris aryl salts. In the above and subsequent definitions, the prefix "hetero" is meant to include linear or cyclic organic radicals having incorporated therein at least one non-carbon and non-hydrogen atom, and is not meant to be limited to the specific examples contained herein. According to U.S. Pat. No. 4,977,198, the onium salts are well known, particularly for use in catalyzing cure of epoxy functional materials.
As disclosed in U.S. Pat. No. 4,279,717, the radiation-initiated cure of epoxysilicones coated on a substrate can be achieved with UV lamps such as: mercury arc lamps, (high, medium and low pressure), Xenon arc lamps, high intensity halogen-tungsten arc lamps, microwave driven arc lamps and lasers.
Thermal-cure silicone release systems as in Sandford, U.S. Pat. No. 4,123,604 and Keil, U.S. Pat. No. 3,527,659 make use of silicone resins, specifically Q-containing resins, to provide tight release for CRAs. Alternative compositions described as being useful CRAs are limited to solvent-dispersed products, such as disclosed in Morrow, U.S. Pat. No. 4,171,397, Reusser, U.S. Pat. No. 4,261,876 and Hockemeyer, U.S. Pat. No. 4,208,504. UV cure epoxysilicone release agents, such as disclosed by Eckberg et al., U.S. Pat. No. 4,279,717 are premium release agents, particularly when applied to film or plastic substrates.
CRAs for epoxysilicone systems have been produced by syntheses of epoxysilicone polymers incorporating polar phenolic and benzoate ester functional groups in the polymer along with reactive cycloaliphatic epoxy groups, as disclosed by Eckberg et al. in U.S. Pat. Nos. 4,952,657 and 4,977,198, respectively. These compositions, however, have proven to be slower-curing than the low release epoxysilicones with which they are to be blended, and are generally not useful at concentrations below 70 wt % of the total coating bath, particularly against non-aggressive rubber or hot melt adhesives.
A major drawback to the use of the "onium" salt catalysts in the polymerization of epoxysilicones lies in the highly polar nature of these salts. As the commonly used silicones are based on non-polar polydimethylsiloxane polymers, the polar "onium" catalysts are not sufficiently miscible with the resin to affect as fast a cure rate as would generally be desirable nor are suspensions of the insoluble catalysts stable. The need therefore exists to devise novel materials and processes in which the miscibility of the photo-initiators and siloxanes are much improved.
Two general approaches have been taken to increase the miscibility of an onium photo-initiator and an epoxysilicone resin. The first approach has been to increase the hydrophobicity of the catalyst through use of onium salts containing non-polar, organic radicals. This approach led to investigations of potential onium salts, particularly long-chain alkyl-substituted bisaryliodonium salts, which are less polar in nature than their sulfonium counterparts. As disclosed in U.S. Pat. Nos. 4,882,201 and 4,279,717, particularly useful catalysts of this type are the linear or branched, C.sub.8 or greater alkyl and alkoxy, mono- or disubstituted, bisaryliodonium salts. As further disclosed in U.S. Pat. No. 4,882,201, the long-chain, alkoxy-substituted aryliodonium salts also possesses the useful property of being much less toxic than the non-substituted onium salt photo-initiators.
A second approach to alleviating the aforementioned miscibility problem between the photo-initiator and a silicone has been to incorporate silphenylene blocks into a siloxane backbone, for example as disclosed in U.S. Pat. No. 4,990,546. This approach, when coupled with the use of the above-described substituted onium salts, proved useful in increasing photo-initiated cure. However, the incorporation of silphenylene blocks into a silicone resin is not commercially viable for release coating applications, since the disilyl-functional benzenes needed to produce the silphenylene-containing polymers are not available in commercial quantities.
In a more indirect effort to overcome the relatively slow cure rates due to the above-mentioned miscibility problem, an epoxysilicone polymer is pre-crosslinked as disclosed, for example, in U.S. Pat. No. 4,987,158. While such pre-crosslinked epoxysilicone networks, formed from vinyl tetramer and SiH-containing linear silicones, partially overcome some of the slow cure associated with long chain epoxysilicone coatings which are not rich in epoxy content, these partially-cured resins still do not possess a solubility with iodonium catalysts that is sufficiently high to be commercially useful as UV-curable materials in most applications.
Attempts have been made to utilize silicone resins with linear UV-curable epoxysilicone fluids for CRA applications. These have been unsuccessful because non-functional Q resins do not cure into the UV-cured release coating, and epoxy-functional Q resins thus far produced are high viscosity materials or solids in the neat form which are immiscible with known premium release silicone polymers without use of solvent carriers.
Due to the above-mentioned considerations, it has therefore been desirable to increase the miscibility of polar compounds, particularly photo-initiator salts, in epoxysilicone resins such that high and efficient cure rates can be economically achieved. In addition, it would also be advantageous to, at the same time, provide for epoxysilicones that are useful as CRAs and in particular are effective to control the release properties over a broad range of values.